1. Field of the Invention
The present invention relates to a method for manufacturing a wiring substrate configured such that a conductor layer of a predetermined pattern is formed on a resin dielectric layer, and an upper resin dielectric layer is formed on the resin dielectric layer and on the conductor layer, and more particularly to a method for manufacturing a wiring substrate configured such that a conductor layer is formed through copper plating on a resin dielectric layer whose surface has been roughened to a predetermined roughness and to which palladium adheres.
2. Description of the Related Art
Conventionally, a wiring substrate has been known configured such that a conductor layer of a predetermined pattern is formed on a surface-roughened resin dielectric layer, and an upper resin dielectric layer is formed on the resin dielectric layer and on the conductor layer.
An example of such a wiring substrate is a wiring substrate 101, which is schematically shown in the partial, enlarged sectional view of FIG. 6. The wiring substrate 101 includes a surface-roughened resin dielectric layer 103. A conductor layer 107 of a predetermined pattern, which includes wiring and pads, is formed on the resin dielectric layer 103 through electroless copper plating and copper electroplating. An upper resin dielectric layer 105 is formed on the resin dielectric layer 103 and on the conductor layer 107.
The wiring substrate 101 is manufactured, for example, in the following manner. A substrate 111 having the resin dielectric layer 103 is prepared. The surface of the resin dielectric layer 103 is roughened to a predetermined roughness by etching (see FIG. 7) for the purpose of enhancing bonding strength between the resin dielectric layer 103, and the conductor layer 107 or the upper resin dielectric layer 105, which are to be formed on the resin dielectric layer 103.
Subsequently, as shown in FIG. 7, palladium 113, which serves as a catalytic metal in the course of formation of an electroless copper plating layer, is formed on the roughened surface of the resin dielectric layer 103.
Next, the substrate 111 to which palladium 113 adheres is subjected to electroless copper plating, thereby forming an electroless copper plating layer 115, as represented by the thick line in FIG. 8, on substantially the entire surface of the resin dielectric layer 103.
The substrate 111 on which the electroless copper plating layer 115 has been formed is heated at 120xc2x0 C. for 60 minutes for the purpose of, for example, drying the wet substrate 111 and enhancing bonding strength between the resin dielectric layer 103 and the electroless copper plating layer 115.
Subsequently, a plating resist layer 117 is formed in a predetermined pattern on the electroless copper plating layer 115 (see FIG. 8).
Subsequently, the substrate 111 on which a plating resist layer 117 has been formed is subjected to copper electroplating, thereby forming a copper electroplating layer 119 on the electroless copper plating layer 115 exposed through the plating resist layer 117 as shown in FIG. 8.
Next, the plating resist layer 117 is removed.
Subsequently, the substrate 111, from which the plating resist layer 117 has been removed, is heated at 150xc2x0 C. for 120 minutes to enhance bonding between the resin dielectric layer 103 and the electroless copper plating layer 115 and between the electroless copper plating layer 115 and the copper electroplating layer 119.
Subsequently, as shown in FIG. 9, the electroless copper plating layer 115 is etched away, except where the same is overlaid with the copper electroplating layer 119, thereby forming the conductor layer 107 in a predetermined pattern. In this case, a metallic residue 121 of the palladium 113 and copper may be present on a portion (hereinafter also called xe2x80x9can exposed portionxe2x80x9d) of the resin dielectric layer 103 which is exposed through the conductor layer 107, such as through spaces between conductor lines.
Then, the substrate 111 is cleaned using a permanganic acid solution, thereby completely removing the metallic residue 121 from the exposed portion of the resin dielectric layer 103.
Subsequently, the upper resin dielectric layer 105 is formed on the resin dielectric layer 103 and on the conductor layer 107, thereby completing the wiring substrate 101 shown in FIG. 6.
3. Problems to be Solved by the Invention
However, permanganic acid treatment for removing the metallic residue 121 is adapted to melt the surface of the resin dielectric layer 103 so as to remove the metallic residue 121 together with resin. Therefore, the treatment roughens the surface of the resin dielectric layer 103 (see FIG. 6), to thereby impair bonding strength between the resin dielectric layer 103 and the upper resin dielectric layer 105.
If permanganic acid treatment is not performed, the upper resin dielectric layer 105 is formed while the metallic residue 121 of the palladium 113 and copper is present on the exposed portion of the resin dielectric layer 103, potentially raising a problem such as a short circuit in the wiring substrate 101 or a reduction in insulation resistance.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide a method for manufacturing a wiring substrate, wherein a conductor layer of a predetermined pattern is formed, through copper plating, on a resin dielectric layer having a roughened surface to which palladium adheres, and an upper resin dielectric layer is formed on the resin dielectric layer and the conductor layer. The method can remove residual substances such as palladium from an exposed portion of the resin dielectric layer, and can establish reliable bonding strength between the resin dielectric layer and the upper resin dielectric layer.
The above object of the present invention has been achieved by providing a method for manufacturing a wiring substrate including a resin dielectric layer, a conductor layer of a predetermined pattern formed on the resin dielectric layer, and an upper resin dielectric layer formed on the resin dielectric layer and on the conductor layer, comprising: a conductor layer forming step of forming the conductor layer, through electroless copper plating and copper electroplating, on the resin dielectric layer of the substrate, the resin dielectric layer having a surface which has been surface-roughened to a predetermined roughness and to which palladium adheres; a cyan treatment step of cleaning the substrate, on which the conductor layer has been formed, using a cyanide-containing solution; and an upper resin dielectric layer forming step of forming an upper resin dielectric layer on the resin dielectric layer and the conductor layer of the cyan-treated substrate.
As mentioned previously, if permanganic acid treatment is used to remove an adhering metallic residue of palladium and copper from a portion (hereinafter also called xe2x80x9can exposed portionxe2x80x9d) of the resin dielectric layer which is exposed through the conductor layer, such as through spaces between conductor lines, the resin dielectric layer is also roughened. This results in impaired bonding strength between the resin dielectric layer and the upper resin dielectric layer.
By contrast, the present invention employs a cyan treatment in place of permanganic acid treatment; i.e., the substrate is cleaned using a cyanide-containing solution. Cleaning with a cyanide-containing solution can remove a metallic residue of palladium and copper without roughening the resin dielectric layer, thereby preventing a problem such as a short circuit in the wiring substrate or a reduction in insulation resistance, while establishing reliable bonding strength between the resin dielectric layer and the upper resin dielectric layer.
A cyanide-containing solution is an aqueous solution of an inorganic cyanide compound such as potassium cyanide or sodium cyanide, or a solution containing an inorganic cyanide compound serving as a predominant component and additives such as an oxidizing agent and a pH adjusting agent.
Preferably, in the cyan treatment step, the substrate is cleaned a plurality of times using the cyanide-containing solution, and dried between cleanings.
According to the present invention, after the substrate is cleaned with the cyanide-containing solution, the substrate is dried and is then cleaned again with a cyanide-containing solution. If needed, this cleaning-and-drying step is repeated. By cleaning the substrate repeatedly with an intermediate drying step, an adhering metallic residue of palladium and copper can be more reliably removed from an exposed portion of the resin dielectric layer as compared with the case where the substrate is cleaned for a long period of time without intermediate drying, thereby more reliably preventing a problem such as a short circuit in the wiring substrate.
Preferably, the substrate is maintained at a temperature not higher than about 85xc2x0 C. in the course of working ranging from the conductor layer forming step to the cyan treatment step.
As mentioned previously, the conventional manufacturing method heats the substrate twice at high temperature in the course of working ranging from the conductor layer forming step to the cyan treatment step. Specifically, after electroless copper plating is performed, the substrate is heated at about 120xc2x0 C. for 60 minutes; and after the plating resist layer is removed, the substrate is heated at about 150xc2x0 C. for 120 minutes. These heat treatment steps are adapted to enhance bonding between the resin dielectric layer and the electroless copper plating layer, and between the electroless copper plating layer and the copper electroplating layer. However, enhancement of bonding between the resin dielectric layer and the electroless copper plating layer fosters the tendency toward the presence of a metallic residue of palladium and copper on an exposed portion of the resin dielectric layer in the step of forming the conductor layer. Since such metallic residue adheres to the resin dielectric layer with a relatively strong force, reliable removal of the metallic residue by cyan treatment may become difficult.
By contrast, according to the present invention, the substrate is maintained at a temperature not higher than about 85xc2x0 C. in the course of working ranging from the conductor layer forming step to the cyan treatment step; i.e., over a span of working between adhesion of palladium to the resin dielectric layer and cleaning of the substrate with a cyanide-containing solution. Thus, in the step of forming the conductor layer, the amount of a metallic residue of palladium and copper present on an exposed portion of the resin dielectric layer is small. Further, strength of adhesion of the metallic residue to the resin dielectric layer is rather low. Therefore, in the cyan treatment step, the metallic residue can be more reliably removed from the exposed portion, thereby more reliably preventing a problem such as a short circuit in the wiring substrate.
Preferably, the above-described method for manufacturing a wiring substrate further comprises a heat treatment step conducted after the cyan treatment step and before the upper resin dielectric layer forming step, for heating the cyan-treated substrate to a temperature higher than about 85xc2x0 C.
According to the present invention, after cyan treatment, the substrate is subjected to heat treatment for heating the substrate at a temperature higher than about 85xc2x0 C., thereby enhancing bonding between the resin dielectric layer and the electroless copper plating layer, and between the electroless copper plating layer and the copper electroplating layer. Since this heat treatment is carried out after cyan treatment, the tendency toward presence of a metallic residue of palladium and copper on an exposed portion of the resin dielectric layer, and the difficulty in removing the metallic residue can both be avoided, which would otherwise occur if heat treatment were carried out prior to cyan treatment.
Preferably, the above-described method for manufacturing a wiring substrate further comprises conducting a conductor roughening step after the heat treatment step and before the upper resin dielectric layer forming step, for roughening the surface of the conductor layer.
In some cases, in order to enhance bonding strength between the conductor layer and the upper resin dielectric layer, the surface of the conductor layer may be roughened. If surface-roughening of the conductor layer is performed before the heat treatment step, the surface of the conductor layer may fail to be roughened to a predetermined roughness.
According to the present invention, after heat treatment, the surface of the conductor layer is roughened. Since the conductor layer is softened (modified) by heating, the surface of the conductor layer can be reliably roughened to a predetermined roughness, whereby bonding strength between the conductor layer and the upper resin dielectric layer can be enhanced.
The present invention provides another method for manufacturing a wiring substrate including a resin dielectric layer, a conductor layer formed in a predetermined pattern on the resin dielectric layer, and an upper resin dielectric layer formed on the resin dielectric layer and on the conductor layer, comprising: an electroless copper plating step of forming an electroless copper plating layer, through electroless copper plating, on the resin dielectric layer of the substrate, the resin dielectric layer having a surface which has been surface-roughened to a predetermined roughness and to which palladium adheres; a drying step of drying the substrate on which the electroless copper plating layer has been formed, at a temperature not higher than about 85xc2x0 C.; a plating resist layer forming step of forming, after the drying step, a plating resist layer of a predetermined pattern on the electroless copper plating layer; a copper electroplating step of forming a copper electroplating layer, through copper electroplating, on the electroless copper plating layer exposed through the plating resist layer; a plating resist layer removing step of removing, after the copper electroplating step, the plating resist layer; an etching-away step of etching away the electroless copper plating layer except where the same is overlaid with the copper electroplating layer, thereby forming the conductor layer; a cyan treatment step of cleaning the substrate, on which the conductor layer has been formed, using a cyanide-containing solution; and an upper resin dielectric layer forming step of forming an upper resin dielectric layer on the resin dielectric layer and the conductor layer of the cyan-treated substrate.
According to the present invention, after an exposed portion of the electroless copper plating layer is etched away, in place of conventionally practiced permanganic acid treatment, cyan treatment is performed for cleaning the substrate using a cyanide-containing solution. Cleaning with a cyanide-containing solution can remove a metallic residue of palladium and copper without roughening the resin dielectric layer, thereby preventing a problem such as a short circuit in the wiring substrate or a reduction in insulation resistance and establishing reliable bonding strength between the resin dielectric layer and the upper resin dielectric layer.
The present invention replaces the conventional step of heating, after electroless copper plating, the substrate at high temperature (at 120xc2x0 C. for 60 minutes) with the step of heating the substrate at a relatively low temperature not higher than about 85xc2x0 C. for drying the substrate. Therefore, the resin dielectric layer and the electroless copper plating layer are bonded with rather low strength. Thus, in the etching-away step, removal of the electroless copper plating layer and palladium is facilitated, thereby reducing the amount of a metallic residue of palladium and copper remaining on an exposed portion of the resin dielectric layer. In the subsequent cyan treatment step, removal of the metallic residue remaining on the exposed portion is facilitated, thereby more reliably preventing a problem such as a short circuit in the wiring substrate.
Preferably, in the cyan treatment step, the substrate is cleaned a plurality of times using the cyanide-containing solution and dried between cleanings.
According to the present invention, after the substrate is cleaned using a cyanide-containing solution, the substrate is dried and is then cleaned again using the cyanide-containing solution. If needed, this cleaning-and-drying step is repeated. By cleaning the substrate repeatedly using a cyanide-containing solution with an intermediate drying step, an adhering metallic residue of palladium and copper can be more reliably removed from an exposed portion of the resin dielectric layer as compared with the case where the substrate is cleaned for a long period of time without intermediate drying, thereby more reliably preventing a problem such as a short circuit in the wiring substrate.
Preferably, the above-described method for manufacturing a wiring substrate further comprises conducting a heat treatment step after the cyan treatment step and before the upper resin dielectric layer forming step, for heating the cyan-treated substrate to a temperature higher than about 85xc2x0 C.
According to the present invention, the substrate is subjected to heat treatment for heating the substrate at a temperature higher than about 85xc2x0 C., thereby enhancing bonding between the resin dielectric layer and the electroless copper plating layer, and between the electroless copper plating layer and the copper electroplating layer. Since this heat treatment is carried out after cyan treatment, the tendency toward presence of a metallic residue of palladium and copper on an exposed portion of the resin dielectric layer, and the difficulty in removing the metallic residue can both be avoided, which would otherwise occur if heat treatment were carried out prior to cyan treatment.
Preferably, the above-described method for manufacturing a wiring substrate further comprises conducting a conductor roughening after the heat treatment step and before the upper resin dielectric layer forming step, for roughening the surface of the conductor layer.
If the conductor roughening step for enhancing bonding strength between the conductor layer and the upper resin dielectric layer is performed before the heat treatment step, the surface of the conductor layer may fail to be roughened to a predetermined roughness.
According to the present invention, after heat treatment, the surface of the conductor layer is roughened. Since the conductor layer is softened (modified) by heating, the surface of the conductor layer can be reliably roughened to a predetermined roughness, whereby bonding strength between the conductor layer and the upper resin dielectric layer can be enhanced.